Seasonal affective disorder

n. SADS, depression that recurs during the fall or winter months as the days grow shorter; syndrome characterised by depressions that repeat once a year at the same (usually during the fall or winter months)

Seasonal affective disorder, also known as winter depression, is an affective, or mood, disorder. Most SAD sufferers experience normal mental health throughout most of the year, but experience depressive symptoms in the winter or summer. The condition in the summer is often referred to as Reverse Seasonal Affective Disorder.
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Seasonal affective disorder is a form of depression most often associated with the lack of daylight in [more]Seasonal affective disorder - Community and Resources

Seasonal affective disorder (SAD) is a form of depression that appears related to fluctuations in the exposure to natural light. It usually strikes during autumn and often continues through the winter when natural light is reduced. Researchers have found that people who have SAD can be helped with the symptoms of their illness if they spend blocks of time bathed in light from a special full-spectrum light source, called a "light box."